1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter device which may be used to take out a signal within a particular frequency band. More specifically, the invention pertains to a surface acoustic wave filter device having highly improved functional characteristics, e.g., temperature characteristic, insertion loss characteristic, phase characteristic, etc.
2. Description of the Prior Art
Conventionally, there is known a surface acoustic wave filter device which includes a piezoelectric substrate having a surface which is formed with interdigital-type transducers (IDT) on input and output sides. Such a surface acoustic wave filter device typically incorporates unidirectional transducers as the IDT's on the input and output sides, thereby to minimize the insertion loss of the device.
This type of unidirectional transducer is disclosed in for examples, JP-B-3-20929, and includes a piezoelectric substrate which comprises a LiNbO.sub.3 monolithic body having a high electromechanical coupling factor, and positive and negative electrodes formed on the substrate and arranged in an interdigital manner. The electrode fingers of the positive and negative electrodes, i.e., those regions of the electrodes which are in overlapping relationship with each other when observed in the propagation direction of the surface acoustic wave, are spaced from each other by a center distance of .lambda./2 wherein .lambda. is the wavelength of a fundamental surface acoustic wave. Further, floating electrodes are formed between adjacent positive and negative electrodes and maintained in electrically floated state. As measured in the propagation direction of the surface acoustic wave, each of the electrode fingers of the positive and negative electrodes and the floating electrodes has a width of .lambda./8, and the center distance "d" between each floating electrode and an adjacent positive or negative electrode is set to satisfy the condition .lambda./8&lt;d&lt;.lambda./4.
Furthermore, another type of surface acoustic wave filter device is disclosed in JP-A-3-133209, which also uses LiNbO.sub.3 as the piezoelectric substrate, and which is formed as a wide band filter device. In this instance, the LiNbO.sub.3 substrate is formed with positive and negative electrodes, and open-type floating electrodes and short-circuited floating electrodes both of which are arranged between the positive and negative electrodes. The width of the electrode finger of each electrode is set at .lambda./12, and the floating electrodes of both types are arranged at equal intervals of .lambda./6 between the positive and negative electrodes.
The surface acoustic wave filter device described above proved to be highly useful due to a relatively small insertion loss, and phase- and frequency-characteristics which can be suitably controlled. On the other hand, as a result of recent progressive development of digital communication systems, there arose a strong demand in the marketplace for surface acoustic wave filter devices having a narrow band characteristic with a low insertion loss. The narrow band characteristic of the filter device requires a stable pass band characteristic with a smaller variation of the pass band for temperature change. Such variation of the pass band is generally not a detrimental problem in the case of wide band filter devices because, even when the pass band is varied with the temperature change, the variation rate of the pass band relative to the entire band width is less significant as the pass band width itself is wide. However, in the case of narrow band filter devices, the pass band width itself is narrow and the variation rate of the pass band width relative to the pass band width is more significant, such that the predetermined center frequency tends to be readily excluded out of the pass band even with a slight temperature change. It is therefore highly desirable for the narrow band filter devices for digital communication systems to have a stable pass band characteristic for the temperature change.
In the surface acoustic wave filter device in which unidirectional transducers are formed on the LiNbO.sub.3 substrate, a higher electromechanical coupling factor of the LiNbO.sub.3 substrate makes it possible to lower the insertion loss of the device. However, an actual situation is that, due to larger pass band variation and band width fluctuation as a result of temperature change, the prior art surface acoustic wave filter device cannot be practically used for the narrow band filter device as it is.